[linux-lvm] Is LVM safe?

[linux-lvm] Is LVM safe?

[John Rowe]

With a 'standard' disk partition (ext3, etc.) I can be confident that if
I have the partition I have the data: I can always just move the disk to
another system and mount it. Is the same true for LVM? 

I am concerned that the LVM system also seems to need some information
kept on the host files system in /etc/lvm* and that if I lose that I'm
in serious trouble.

John

Re: [linux-lvm] Is LVM safe?

[Rainer Krienke]

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On Montag 20 Juni 2005 14:02, John Rowe wrote:
> With a 'standard' disk partition (ext3, etc.) I can be confident that if
> I have the partition I have the data: I can always just move the disk to
> another system and mount it. Is the same true for LVM?
>
> I am concerned that the LVM system also seems to need some information
> kept on the host files system in /etc/lvm* and that if I lose that I'm
> in serious trouble.

The main difference between a partition and lvm is in my eyes that in lvm you 
usually do not have a single disk where your data is on. Instead you will 
have a bunch of disks and possibly your data are spread across all of these 
disks (depending on the setup of your volumegroups). 
 
So if you want to move your disk to another system you will have to move all 
disks that have been part of the lvm structure. Since lvm scans all disks to 
build up the volumegroups and logical volumes it should also work after you 
moved your disks to another system. 

If you modified you /etc/lvm/lvm.conf file you probably will have to take a 
look at this file on the old and new system to get things right since you can 
for example determine which of the systems disks are scanned for physical 
volumes and volumegroups. Since there might be a different set of disks in 
the old and new system (aside from the disks you moved) the configuration 
could need an update in this case.

Have a nice day
Rainer
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Re: [linux-lvm] Is LVM safe?

[Erik Ohrnberger]

On Mon, June 20, 2005 9:17, Rainer Krienke said:
>
> The main difference between a partition and lvm is in my eyes that in lvm
> you
> usually do not have a single disk where your data is on. Instead you will
> have a bunch of disks and possibly your data are spread across all of
> these
> disks (depending on the setup of your volumegroups).
> (snip)

Hmm.
The thought just crossed my mind that the failure rate of the LVM would be
the sum of the failure rates of all the disks in your LVM.

Re: [linux-lvm] Is LVM safe?

[Andy Smith]

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On Mon, Jun 20, 2005 at 09:26:49AM -0400, Erik Ohrnberger wrote:
> The thought just crossed my mind that the failure rate of the LVM would be
> the sum of the failure rates of all the disks in your LVM.

You can run LVM on top of software RAID... (or indeed hardware RAID
of course)

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Re: [linux-lvm] Is LVM safe?

[John Rowe]

Thanks for the reply but this doesn't answer my question:

If I have a Physical Volume which is the only Physical Volume in a
Volume Group, can I always recover the recover the Volume Group if I
have absolutely nothing else just like I can with an ext3 partition?

It seems to me that the only possible answers are "yes" or "no", with
"yes, but it's difficult" somewhere in between!

Thanks again.

John

Re: [linux-lvm] Is LVM safe?

[Philipp Riegger]

On 20. Jun 2005, at 15:49 Uhr, John Rowe wrote:

> If I have a Physical Volume which is the only Physical Volume in a
> Volume Group, can I always recover the recover the Volume Group if I
> have absolutely nothing else just like I can with an ext3 partition?

Yes, it's easy. You just need a kernel with DM-Support and the 
LVM2-Tools.

A simple

vgscan
vgchangs -ay

And you have your "Partitions" back.

Re: [linux-lvm] Is LVM safe?

[John Rowe]

Thanks, this is more or less what I've been trying:

#vgscan --mknodes -vv
      Setting global/locking_type to 1
      Setting global/locking_dir to /var/lock/lvm
      File-based locking enabled.
    Wiping cache of LVM-capable devices
    Wiping internal cache
  Reading all physical volumes.  This may take a while...
    Finding all volume groups
      /dev/md0: size is 0 sectors
      /dev/hda1: size is 417627 sectors
      /dev/hda1: No label detected
      /dev/hda2: size is 40965750 sectors
      /dev/hda2: No label detected
      /dev/hda3: size is 6136830 sectors
      /dev/hda3: No label detected
      /dev/hda4: size is 265056435 sectors
      /dev/hda4: lvm2 label detected
  /dev/cdrom: open failed: No medium found
    Finding all logical volumes


# vgchange -ay -vv
      Setting global/locking_type to 1
      Setting global/locking_dir to /var/lock/lvm
      File-based locking enabled.
    Finding all volume groups
      /dev/hda1: No label detected
      /dev/hda2: No label detected
      /dev/hda3: No label detected
      /dev/hda4: lvm2 label detected


But it doesn't seem to find anything!

John

Re: [linux-lvm] Is LVM safe?

[AJ Lewis]

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On Mon, Jun 20, 2005 at 04:09:10PM +0200, Philipp Riegger wrote:
> On 20. Jun 2005, at 15:49 Uhr, John Rowe wrote:
> 
> >If I have a Physical Volume which is the only Physical Volume in a
> >Volume Group, can I always recover the recover the Volume Group if I
> >have absolutely nothing else just like I can with an ext3 partition?
> 
> Yes, it's easy. You just need a kernel with DM-Support and the 
> LVM2-Tools.
> 
> A simple
> 
> vgscan
> vgchangs -ay
> 
> And you have your "Partitions" back.

Well, it's not quite that simple - if something scribbles over your LVM
metadata (i've never seen this happen where it wasn't user error), you need to
do quite a bit more work to get things back so you can activate your LVs so
you can fsck your ext3 partition.  It usually involves at least running a
vgcfgrestore against the backup file in /etc/lvm/backup.  So yes, it can be
more complicated if the lvm metadata gets broken - but this is similar to what
happens if the partition table gets busted on a normal disk.

-- 
AJ Lewis                                   Voice:  612-638-0500
Red Hat Inc.                               E-Mail: alewis@redhat.com
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Re: [linux-lvm] Is LVM safe?

[John Rowe]

> > Yes, it's easy. You just need a kernel with DM-Support and the 

> But this is similar to what
> happens if the partition table gets busted on a normal disk.

So the previous poster's "Yes" actually means "No". And a broken
partition table will zap a disk with an LVM partition too won't it? So
the bottom line is that if I want to move an LVM partition to a
different system I have to hold my breath and pray. 

The idea that a disk partition needs Unix file to be able to read it is
absolutely astonishing.

John

Re: [linux-lvm] Is LVM safe?

[AJ Lewis]

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On Mon, Jun 20, 2005 at 04:00:49PM +0100, John Rowe wrote:
> 
> > > Yes, it's easy. You just need a kernel with DM-Support and the 
> 
> > But this is similar to what
> > happens if the partition table gets busted on a normal disk.
> 
> So the previous poster's "Yes" actually means "No". And a broken
> partition table will zap a disk with an LVM partition too won't it? So
> the bottom line is that if I want to move an LVM partition to a
> different system I have to hold my breath and pray. 

No - that should just work.  You just need to have the device-mapper kernel
modules and the lvm2 tools.  Then you do a 'vgscan && vgchange -ay' and it
will be there.
 
> The idea that a disk partition needs Unix file to be able to read it is
> absolutely astonishing.

What?

-- 
AJ Lewis                                   Voice:  612-638-0500
Red Hat Inc.                               E-Mail: alewis@redhat.com
One Main Street SE, Suite 209
Minneapolis, MN 55414
   
Current GPG fingerprint = D9F8 EDCE 4242 855F A03D  9B63 F50C 54A8 578C 8715
Grab the key at: http://people.redhat.com/alewis/gpg.html or one of the
many keyservers out there...


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Re: [linux-lvm] Is LVM safe?

[John Rowe]

>  
> > The idea that a disk partition needs Unix file to be able to read it is
> > absolutely astonishing.
> 
> What?

I'm getting two different answers here. One person says I don't
need /etc/lvm.*, the other says I do!

FWIW, I'm running Linux version 2.6.9-5.0.5.EL having 'upgraded' from
RedHat9 to Scientific Linux 4 (a RedHat Enterprise clone), and I can't
see my LVM..

Of course I have backups but it worries me for the future.

John

Re: [linux-lvm] Is LVM safe?

[AJ Lewis]

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On Mon, Jun 20, 2005 at 04:27:35PM +0100, John Rowe wrote:
> 
> >  
> > > The idea that a disk partition needs Unix file to be able to read it is
> > > absolutely astonishing.
> > 
> > What?
> 
> I'm getting two different answers here. One person says I don't
> need /etc/lvm.*, the other says I do!

Ok, so the only time you *need* /etc/lvm/* is if you have to recover from bad
metadata.  Every PV in your VG has a complete copy of the metadata for that
VG, so you have multiple copies of the layout - if you have one of these, you
can recover your LVM setup.
 
> FWIW, I'm running Linux version 2.6.9-5.0.5.EL having 'upgraded' from
> RedHat9 to Scientific Linux 4 (a RedHat Enterprise clone), and I can't
> see my LVM..

Seeing the output of 'vgscan -vvv' and 'vgchange -ay -vvv' would be helpful -
maybe you could toss it up on pastebin.com?
 
> Of course I have backups but it worries me for the future.

-- 
AJ Lewis                                   Voice:  612-638-0500
Red Hat Inc.                               E-Mail: alewis@redhat.com
One Main Street SE, Suite 209
Minneapolis, MN 55414
   
Current GPG fingerprint = D9F8 EDCE 4242 855F A03D  9B63 F50C 54A8 578C 8715
Grab the key at: http://people.redhat.com/alewis/gpg.html or one of the
many keyservers out there...


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Re: [linux-lvm] Is LVM safe?

[Eric Hopper]

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On Mon, Jun 20, 2005 at 04:00:49PM +0100, John Rowe wrote:
> 
> > > Yes, it's easy. You just need a kernel with DM-Support and the 
> 
> > But this is similar to what
> > happens if the partition table gets busted on a normal disk.
> 
> So the previous poster's "Yes" actually means "No". And a broken
> partition table will zap a disk with an LVM partition too won't it? So
> the bottom line is that if I want to move an LVM partition to a
> different system I have to hold my breath and pray.

No, it means "Yes" just like he said.  There is a condition that you
didn't explicitly state in the original question.  If the LVM metadata
is messed up somehow (which doesn't generally happen unless you do
something stupid), you can't recover the LVM volumes without a
filesystem backup of the metadata.

> The idea that a disk partition needs Unix file to be able to read it is
> absolutely astonishing.

Umm, try this sometime...

dd if=/dev/zero of=/dev/hda bs=512 count=1

Then just try to resurrect the partition.

Unless you've created a backup of your partition table into a Unix file,
then you aren't going to recover the partition table, period.

So, poof, there, you have a disk partition that needs a Unix file to be
able to read it.

Since LVM maintains backups of all of this metadata in files, it's
signficantly better for recovery than Unix partitions are.

*sigh*,
-- 
"It does me no injury for my neighbor to say there are twenty gods or no God.
It neither picks my pocket nor breaks my leg."  --- Thomas Jefferson
"Go to Heaven for the climate, Hell for the company."  -- Mark Twain
-- Eric Hopper (hopper@omnifarious.org  http://www.omnifarious.org/~hopper) --

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